1. Field of the Invention
The present invention relates to an optical module and a manufacturing method thereof.
2. Background Art
Conventionally, as is disclosed, for example, in Japanese Patent Laid-Open No. 5-127050, an optical module is known which is provided with a lens cap combining a plurality of parts having mutually different linear expansion coefficients. The lens cap is a part that holds a lens. According to this Patent document, as measures for thermal fatigue, a lens holder contacting the lens is formed of pure iron and a holding part that holds this lens holder is formed of Kovar (registered trademark), and these components are soldered into one lens cap.
According to such a configuration, the lens holder has a larger linear expansion coefficient than that of the holding part. Thus, the lens cap has a two-division structure, that is, the lens cap is constructed of two parts and the two parts are formed of mutually different materials. For a package connection part, the same material as the package material is used and for a lens connection part, a material having a thermal expansion coefficient similar to that of the lens is used.
Other prior art includes Japanese Patent Laid-Open No. 5-127050, Japanese Patent Laid-Open No. 2009-37055, Japanese Patent Laid-Open No. 2003-322755, Japanese Patent Laid-Open No. 8-241933.
A lens is attached to a lens cap, for example, by soldering the lens to the lens cap or by molding the lens into the lens cap through press molding. In the latter press molding method in particular, air tightness between the lens cap and the lens can be enhanced through thermal caulking utilizing a difference between a linear expansion coefficient of the lens cap and a linear expansion coefficient of the lens glass.
Lens cap manufacturing steps include steps carried out under a high-temperature atmosphere and press molding of the lens is one of those steps. When combining a plurality of parts having mutually different linear expansion coefficients into one lens cap as in the case of the above-described prior art, it should be noted that the amount of thermal expansion varies among the parts under the high-temperature atmosphere. None of the conventional manufacturing methods attaches importance to this respect and a positional variation of the lens may increase due to the difference in the amount of thermal expansion.
Furthermore, when a plurality of parts having mutually different linear expansion coefficients are combined into one lens cap as in the case of the above-described prior art, scales of gaps among a plurality of parts change due to differences in the amount of thermal expansion. Under such circumstances, the present inventors discovered a technique of improving positioning accuracy utilizing a difference in the amount of thermal expansion among parts under a high-temperature atmosphere by adjusting relationships of linear expansion coefficients among components and shapes of the respective components of the lens cap.